The effect of inoculating endophytic N2-fixing bacteria on micropropagated sugarcane plants

The aim of this work was to evaluate the effect of the inoculation of endophytic N2-fixing bacteria on the development of micropropagated sugarcane plants. The endophytic population of each inoculated species was monitored during the growth period, and biological nitrogen fixation (BNF) contribution of each inoculation treatment was assessed using the 15N-isotope dilution technique. Seven different combinations of inoculum were used, using five endophytic diazotrophic species (Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, Herbaspirillum rubrisubalbicans, Azospirillum amazonense and Burkholderia sp.), originally isolated from sugarcane plants. The results showed a clear physiological effect on the development of the inoculated plants, resulting in alteration of the dry matter-partitioning pattern and increase on root dry matter as compared to uninoculated plants. Indeed, all inoculated diazotrophic species could be reisolated in high numbers from the rhizomes of the inoculated plants, even 400 days after inoculation (DAI), suggesting the establishment of the inoculated bacteria. However, a negative effect of the mixture of all five species on the survival of plantlets was observed 45 days after inoculation, just after acclimatization. The analysis of the BNF contribution using the 15N-isotope dilution technique showed that inoculation promoted some increase in the BNF contribution to the plant tissues. The best treatment was the mixture of all five strains, followed by the treatment with a mixture of Herbaspirillum spp. The contribution was much lower when the plants were inoculated with a mixture of G. diazotrophicus with A. amazonense and Burkholderia sp. A BNF contribution around 30% of total nitrogen accumulated was observed in micropropagated plants inoculated with the mixture of strains, suggesting that the combination of species in the inocula is the best strategy to improve sugarcane crops dependent on the biological nitrogen fixation process.